1. The Field of the Invention
The present invention relates to methods and systems for displaying images with increased resolution, and more particularly, to methods and systems that utilize an increased number of sampling points to generate an increased resolution of an image displayed on a display device, such as a liquid crystal display.
2. The Prior State of the Art
With the advent of the information age, individuals worldwide spend substantial amounts of time viewing display devices and thus suffer from problems such as eyestrain. The display devices that are viewed by the individuals display electronic image data, such as text characters. It has been observed that text is more easily read and eyestrain is reduced as the resolution of text characters improves. Thus, achieving high resolution of text and graphics displayed on display devices has become increasingly important.
One such display device that is increasingly popular is a flat panel display device, such as a liquid crystal display (LCD). However, most traditional image processing techniques, including generating and displaying fonts, have been developed and optimized for display on a cathode ray tube (CRT) display rather than for display on an LCD. Furthermore, existing text display routines fail to take into consideration the unique physical characteristics of flat panel display devices, which differ considerably from the characteristics of CRT devices, particularly in regard to the physical characteristics of the light sources of the display devices.
CRT display devices use scanning electron beams that are controlled in an analog manner to activate phosphor positioned on a screen. A pixel of a CRT display device that has been illuminated by the electron beams consists of a triad of dots, each of a different color. The dots included in a pixel are controlled together to generate what is perceived by the user as a single point or region of light having a selected color defined by a particular hue, saturation, and intensity. The individual dots in a pixel of a CRT display device are not separately controllable. Conventional image processing techniques map a single sample of image data to an entire pixel, with the three dots included in the pixel together representing a single portion of the image. CRT display devices have been widely used in combination with desktop personal computers, workstations, and in other computing environments in which portability is not an important consideration.
In contrast to CRT display devices, the pixels of LCD devices, particularly those that are digitally driven, have separately addressable and separately controllable pixel sub-components. For example, a pixel of an LCD display device may have separately controllable red, green, and blue pixel sub-components. Each pixel sub-component of the pixels of an LCD device is a discrete light emitting device that can be individually and digitally controlled. However, LCD display devices have been used in conjunction with image processing techniques originally designed for CRT display devices, such that the separately controllable nature of the pixel sub-components is not utilized. Existing text rendering processes, when applied to LCD display devices, result in each three-part pixel representing a single portion of the image. LCD devices have become widely used in portable or laptop computers due to their size, weight, and relatively low power requirements. Over the years, however, LCD devices have begun to more common in other computing environments, and have become more widely used with non-portable personal computers.
Conventional rendering processes applied to LCD devices are illustrated in FIG. 1, which shows image data 10 being mapped to entire pixels 11 of a region 12 of an LCD device. Image data 10 and portion 12 of the flat panel display device (e.g., LCD device) are depicted as including corresponding rows R(N) through R(N+2) and columns C(N) through C(N+2). Portion 12 of the flat panel display device includes pixels 11, each of which has separately controllable red, green, and blue pixel sub-components.
As part of the mapping operation, a single sample 14 that is representative of the region 15 of image data 10 defined by the intersection of row R(N) and column C(N+1) is mapped to the entire three-part pixel 11A located at the intersection of row R(N) and column C(N+1). The luminous intensity values used to illuminate the R, G, and B pixel sub-components of pixel 11A are generated based on the single sample 14. As a result, the entire pixel 11A represents a single region of the image data, namely, region 15. Although the R, G, and B pixel sub-components are separately controllable, the conventional image rendering process of FIG. 1 does not take advantage of their separately controllable nature, but instead operates them together to display a single color that represents a single region of the image.
Text characters represent one type of image that is particularly difficult to accurately display given typical flat panel display resolutions of 72 or 96 dots (pixels) per inch (dpi). Such display resolutions are far lower than the 600 dpi resolution supported by most printers. Even higher resolutions are found in most commercially printed text such as books and magazines. As such, not enough pixels are available to draw smooth character shapes, especially at common text sizes of 10, 12, and 14 point type. At such common text rendering sizes, portions of the text appear more prominent and coarse on the display device than in their print equivalent
It would, therefore, be an advancement in the art to improve the resolution of text and graphics displayed on display devices, particularly on flat panel displays. It would be an advancement in the art to reduce the coarseness of displayed images so that they more closely resemble their print equivalents or the font image data designed by typographers. It would also be desirable for the image processing techniques that provide such improved resolution to take into consideration the unique physical characteristics of flat panel display devices.
The present invention is directed to methods and systems for displaying images on a flat panel display device, such as a liquid crystal display (LCD). Flat panel display devices use various types of pixel arrangements, such as horizontal or vertical striping, and the present invention can be applied to any of the arrangement alternatives to provide an increased resolution on the display device.
The invention relates to image processing operations whereby individual pixel sub-components of a flat panel display device are separately controlled and represent different portions of an image, rather than the entire pixel representing a single portion of the image. Unlike conventional image processing techniques, the image processing operations of the invention take advantage of the separately controllable nature of pixel sub-components in LCD display devices. As a result, text and graphics rendered according to the invention have improved resolution and readability.
The invention is described herein primarily in the context of rendering text characters, although the invention also extends to processing image data representing graphics and the like. Text characters defined geometrically by a set of points, lines, and curves that represent the outline of the character represent an example of the types of image data that can be processed according to the invention.
The general image processing operation of the invention includes a scaling operation, a hinting operation and a scan conversion operation that are performed on the image data. Although the scaling operation and the hinting operation are performed prior to the scan conversion operation, the following discussion will be first directed to scan conversion to introduce basic concepts that will facilitate an understanding of the other operations, namely, a supersampling rate and an overscaling factor.
In order to enable each of the pixel sub-components of a pixel to represent a different portion of the image, the scaled and hinted image data is supersampled in the scan conversion operation. The data is xe2x80x9csupersampledxe2x80x9d in the sense that more samples of the image data are generated than would be required in conventional image processing techniques. When the pixels of the display device have three pixel sub-components, the image data will be used to generate at least three samples in each region of the image data that corresponds to an entire pixel. Often, the supersampling rate, or the number of samples generated in the supersampling operation for each region of the image data that corresponds to an entire pixel, is greater than three. The number of samples depends on weighting factors that are used to map the samples to individual pixel sub-components as will be described in greater detail herein. For instance, the image data can be sampled at a supersampling rate of 10, 16, 20 or any other desired number of samples per pixel-sized region of the image data. In general, greater resolution of the displayed image can be obtained as the supersampling rate is increased and approaches the resolution of the image data. The samples are then mapped to pixel sub-components to generate a bitmap later used in displaying the image on the display device
In order to facilitate the supersampling, the image data that is to be supersampled is overscaled in the direction perpendicular to the striping of the display device as part of the scan conversion operation. The overscaling is performed using an overscaling factor that is equal to the supersampling rate, or the number of samples to be generated for each region of the image data that corresponds to a full pixel.
The image data that is subjected to the scan conversion operation as described above is first processed in the scaling operation and the hinting operation. The scaling operation can be trivial, with the image data being scaled by a factor of one in the directions perpendicular and parallel to the striping. In such trivial instances the scaling factor can be omitted. Alternatively, the scaling factor can be non-trivial, with the image data being scaled in both directions perpendicular and parallel to the striping by a factor other than one, or with the image data being scaled by one factor in the direction perpendicular to the striping and by a different factor in the direction parallel to the striping.
The hinting operation involves superimposing the scaled image data onto a grid having grid points defined by the positions of the pixels of the display device and adjusting the position of key points on the image data (i.e., points on a character outline) with respect to the grid. The key points are rounded to grid points that have fractional positions on the grid. The grid points are fractional in the sense that they can fall on the grid at locations other than full pixel boundaries. The denominator of the fractional position is equal to the overscaling factor that is used in the scan conversion operation described above. In other words, the number of grid positions in a particular pixel-sized region of the grid to which the key points can be adjusted is equal to the overscaling factor. If the supersampling rate and the overscaling factor of the scan conversion process is 16, the image data is adjusted to grid points having fractional positions of {fraction (1/16)}th of a pixel in the hinting operation. The hinted image data is then available to be processed in the scan conversion operation described above.
The foregoing scaling, hinting and scan conversion operations enable image data to be displayed at a higher resolution on a flat panel display device, such as an LCD, compared to prior art image rendering processes. Each pixel sub-component represents a spatially different region of the image data, rather than entire pixels representing single regions of the image.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.